O'Hagan R C, Hassell J A
Cancer Research Group, Institute for Molecular Biology and Biotechnology, McMaster University, Hamilton, Ontario, Canada.
Oncogene. 1998 Jan 22;16(3):301-10. doi: 10.1038/sj.onc.1201547.
The HER2/neu gene, which is overexpressed in 20-30% of human breast tumors, encodes a receptor tyrosine kinase that functions through multiple signaling pathways to regulate the activity of nuclear transcription factors. We have reported that PEA3, an Ets family transcription factor, is overexpressed in HER2/Neu-induced breast tumors and their metastases. To account for the increased levels of PEA3 in these tumors we have suggested that HER2/Neu enhances PEA3 transcriptional activity, which then acts to stimulate expression of the PEA3 gene. This hypothesis is consistent with the occurrence of PEA3 binding sites in the PEA3 promoter and with the ability of PEA3 to transactivate this promoter. To learn whether HER2/Neu indeed regulates PEA3 activity we measured the capacity of constitutively-activated HER2/Neu to affect PEA3-dependent reporter gene expression. Coexpression of PEA3 and HER2/Neu stimulated PEA3-dependent reporter gene expression to a much greater extent than did either protein alone suggesting that HER2/Neu upregulates the transcriptional activity of PEA3. To define the pathway whereby HER2/Neu functions we employed dominant-negative mutants of signaling proteins known to be downstream of HER2/Neu. Overexpression of Rap1a, a Ras-related protein capable of antagonizing Ras function, completely inhibited the ability of HER2/Neu to stimulate PEA3-dependent gene expression. Ras is known to stimulate at least two mitogen-activated protein kinase (MAPK) cascades, the extracellular-regulated kinase (ERK) cascade and the stress-activated kinase (SAPK) or Jun kinase (JNK) cascade. Similarly, HER2/Neu activated both ERKs and SAPKs/JNKs in a Ras-dependent fashion. Dominant-inhibitory mutants in either the ERK or SAPK/JNK cascades partially inhibited HER2/Neu activation of PEA3-dependent gene expression. These findings suggest that HER2/Neu regulates PEA3 activity through two different Ras-dependent MAPK pathways.
HER2/neu基因在20%-30%的人类乳腺肿瘤中过度表达，它编码一种受体酪氨酸激酶，该激酶通过多种信号通路发挥作用，以调节核转录因子的活性。我们曾报道，Ets家族转录因子PEA3在HER2/Neu诱导的乳腺肿瘤及其转移灶中过度表达。为了解释这些肿瘤中PEA3水平的升高，我们提出HER2/Neu增强了PEA3的转录活性，进而刺激PEA3基因的表达。这一假设与PEA3启动子中存在PEA3结合位点以及PEA3反式激活该启动子的能力相一致。为了了解HER2/Neu是否确实调节PEA3活性，我们测量了组成型激活的HER2/Neu影响PEA3依赖的报告基因表达的能力。PEA3和HER2/Neu的共表达比单独的任何一种蛋白都能更大程度地刺激PEA3依赖的报告基因表达，这表明HER2/Neu上调了PEA3的转录活性。为了确定HER2/Neu发挥作用的途径，我们使用了已知在HER2/Neu下游的信号蛋白的显性负性突变体。Rap1a是一种能够拮抗Ras功能的Ras相关蛋白，其过表达完全抑制了HER2/Neu刺激PEA3依赖基因表达的能力。已知Ras可激活至少两条丝裂原活化蛋白激酶（MAPK）级联反应，即细胞外调节激酶（ERK）级联反应和应激激活激酶（SAPK）或Jun激酶（JNK）级联反应。同样，HER2/Neu以Ras依赖的方式激活ERK和SAPK/JNK。ERK或SAPK/JNK级联反应中的显性抑制突变体部分抑制了HER2/Neu对PEA3依赖基因表达的激活。这些发现表明，HER2/Neu通过两条不同的Ras依赖的MAPK途径调节PEA3活性。
